This invention relates to intra-ocular lenses (IOL) and, more particularly, to an improved multi-focal intra-ocular lens.
Over the past several decades, it has become a common place surgical procedure to replace an opacified lens in the human eye with an artificial single power IOL. Such replacements have seen wide success. Until recently, the employment of multi-focal IOL's had not been considered seriously. However, with advances in the state of the art in multi-focal contact lenses, physicians are proceeding with the implantation of multi-focal IOL's.
Some of the more successful contact lenses of the multi-focal type are called "simultaneous image lenses". Those lenses are characterized by an aspheric anterior and/or posterior surface and by a continuously changing power from the para-central area to the mid-periphery. Lenses of this type are described in US-A-3,031,917 to Wesley; US-A-3,037,425 to DeCarle and US-A-4,636,049 to Blaker. The Wesley lens includes a small center zone for near vision surrounded by a concentric distance correction zone. The DeCarle lens includes an opposite construct wherein the distance zone is in the center and is surrounded by the near correction zone. Blaker, describes a lens similar to the Wesley lens, however, he indicates that the near zone center section should be approximately equal to half the pupil area of the eye under average light reading conditions. The latter consideration indicates one of the problems with these lenses - i.e. that they are affected by the pupil size in that the pupil must be large enough to let enough light through the higher add zone of the lens to provide true bifocal action. Lenses of the Wesley/Blaker type are called reverse centrad bifocals.
One significant problem with the reverse centrad bifocals is that during outdoor activities in bright light, or in the presence of a bright illumination at night (e.g. such as driving a car in the presence of oncoming traffic), pupillary constriction reduces the proportion and percentage of rays of light that pass through the distance outer zone thus reducing the quality of distance vision. In fact, if there is sufficient pupillary constriction during the day or as a result of the headlights from oncoming vehicles at night, substantially all distance vision may be lost. This is obviously unacceptable - especially when it is considered that such a loss, when driving a car or as a pedestrian, is life threatening.
Recently, Nielsen at the Center for Eye Surgery in Clearwater, Florida has implanted bifocal IOL's employing the designs suggested by Wesley and Blaker. Those lenses were implanted in a number of patients and were reported as providing "successful results". (see Ophthalmology Times volume 11, number 9, May 1, 1986, pages 1, 77 and 78).
Nielsen's implanted lenses experience the same defects as the reverse centrad bifocal lenses, i.e during activities outdoors in bright light or at night when driving a car in oncoming traffic conditions, pupillary constriction reduces the proportion and percentage of rays of light that can be perceived from the distance (outer) zone and thus reduces the quality of distance vision.
German Published Patent Application DE 3332313 A1 (U.S. 4,813,955) describes a multifocal intra-ocular lens wherein the near and far regions of the lens have approximately equal surface proportions and are symmetrically disposed as increasing concentric circles. The patent teaches that the approximate 50/50 ratio of surface areas of near and far correction regions is to be kept constant. This constraint creates problems in low light situations, i.e. at night. As the pupil enlarges, half the light is focused for near vision and half for far vision. This reduces the light utilizable for either far or near vision to one half the available light and significantly reduces the ability to see at night.
If a design is chosen which utilizes a far vision center zone, the lens is restricted to 50% or greater far vision. This design does not offer a combination of dimensions which would allow a more than 50% near vision under preferred reading conditions.
WO-A-86/03961 to Vannas describes in intraocular lens that is constructed with a plurality of concentric zones. Vannas indicates that the center-most correction zone can be for distance vision. One portion of Vannas' lens is comprised of a first material and a second portion of a second material, the refractive coefficients of the two materials differing. Vannas further teaches that the diameter of his lens is apporximately 6 millimeters and the "central part" is 1.8 - 2.3 millimeters. If it is assumed that Vannas' lens is drawn to scale (and there is no such indication in the Vannas document that this is so) an analysis of the lens of Fig. 7 indicates that it exhibits predominantly near zone correction light in both the "too bright" "too dark" regions. The Vannas lens shown in Fig. 10 allows predominantly near zone correction light in the "too dark" region. Neither of Vannas' lenses exhibit the characteristic of enabling distance correction light to predominate in both of the too bright and too dark regions.
The closest prior art is represented by US-A-4,752,123. This discloses a lens having a centre zone with a distance power correction, an inner ring zone with a near power correction and an outer ring zone with a distance power correction. This prior art lens is, however, a contact lens rather than an intra-ocular lens which is the subject of the present invention. Accordingly, although three separate zones are disclosed, as in the present invention, the relative sizes of the zones and their effect on a user will not be at all comparable. Although the prior art device provides that more than 50% of the light entering the user's eye passes through the distance power zones both at high light levels and at low light levels, there is no concern about what happens at intermediate light levels. This means that the prior art contact lens is not ideal for use in intermediate light conditions, where a user wants to undertake close work, for example reading.
It is an object of this invention to provide an improved bifocal intra-ocular lens which preserves distance vision while also enhancing near vision under moderate light conditions.
It is another object of this invention to provide an improved bifocal intra-ocular lens which is particularly adapted to insertion into the posterior lens capsule.
According to the present invention there is provided an intra-ocular lens (10) for eye implantation including a lens body having optical portions comprised of a unitary material and including at least a centre zone (12) having a distance power correction, an inner ring zone (14) having a near power correction, and an outer ring zone (16) having a distance power correction, the lens admitting into a user's eye, in use, a quantity of light of which
- (a) Greater than 50% passes through the centre zone (12) under bright light conditions when the user's pupil diameter is less than about 3mm;
- (b) Greater than 50% passes through the combined areas of the centre zone (12) and the outer ring zone (16) under low light conditions when the user's pupil diameter is greater than about 4mm; and
- (c) Greater than 50% passes through the inner ring zone (14) under moderate light conditions when the user's pupil diameter is between about 3 and 4mm.
The present invention improves vision during moderate light conditions, which are the conditions under which close work such as reading is likely to be undertaken. During such moderate light conditions the user's pupil diameter will be between about 3mm and about 4mm, and more than 50% of the light entering the user's eye then passes through the inner ring zone, which has a near-power correction. A greater proportion of the light is therefore properly focused under such conditions on to the user's retina.
DESCRIPTION OF THE DRAWINGSFig. 1 is a plan view of an IOL embodying the invention.
Fig. 2 is a sectional view of the invention taken along line 2-2.
Fig. 3 is a section view of an eye with the lens of this invention implanted in the posterior lens capsule.
Fig. 4 is a plan view of an IOL embodying the invention with a circular haptic.
Fig. 5 is a plan view of an IOL embodying the invention with enhanced bright and low light distance power corrections.
Fig. 6 is a side view of the lens of Fig. 5 and shows representative dimensions for the lens' correction zones.
Fig. 7 is a plot which shows the dominant affect of the distance correction zones of a lens incorporating the invention.
DETAILED DESCRIPTION OF THE INVENTIONReferring now to Fig. 1, there is shown a plan view of a bifocal IOL particularly adapted to implantation in the posterior lens capsule. Lens body 10 is formed of a single piece of plastic material, such as silicone, PMMA, other acrylates, polycarbonates, hydrogels or similar optically suitable materials. The lens is comprised of three correction zones, a circular zone 12 having a distance power correction; a concentrically arranged near power correction zone 14 and a second concentrically arranged distance power correction zone 16. A pair of haptics 18 and 20 are integrally formed with lens 10 and provide the centering facility for the lens when it is implanted in the posterior lens capsule. In the conventional manner, haptics 18 and 20 are flexible and bear against the inner surfaces of the lens capsule to center lens 10 subsequent to its implantation.
Dotted circles 22, 24 and 26 are representations of average pupillary openings under expected bright light conditions, average light conditions and low light conditions respectively. Pupillary openings 22, 24 and 26 are approximately 2mm, 4mm, and 6mm in diameter. The 2mm dimension is the smallest pupillary opening achieved under extreme bright light conditions or with the use of drugs to restrict the pupil, i.e., Miacol Pilocarpine. Under moderate light conditions the pupil ranges from 2.7 to 4.0mm. The 4mm dimension approximates the largest pupil opening involved for near vision. In dark conditions, the pupil expands beyond 4mm. The 6mm dimension approximates an average pupil in low light conditions. The preferred dimensions of the correction zones of IOL 10 are indicated in the side view of lens 10 in Fig. 2. Center zone 12 is approximately 1.0mm in diameter; first concentric near zone 14 has a preferred radial width in the range of 1.15mm to 2.12mm and the outer diameter of lens 10 has a preferred overall range of from 5mm to 9mm.
With the above noted zone dimensions, it can be seen that under expected bright light conditions, the diameter of zone 12 is less than the expected minimum pupillary diameter 22 and assures continual distance vision. Under low light (dark) conditions, the width of concentric zone 16 is such as to enable substantial amounts of distance light to enter pupillary opening 26. It can further be seen that if the pupil expands further than is shown by dotted line 26, that distance corrected light entering the pupillary opening increases as the square of the radius thus enabling improved distance vision even under low light conditions.
Referring now to Fig. 3, lens 10 is shown implanted in the posterior lens capsule 30. The focal planes for all of the segments of IOL 10 fall on the macular portion of the retina and provide simultaneous images. As shown in Fig. 2, posterior surface 32 of lens 10 has a convex form which conforms to the posterior portion of lens capsule 30 to thereby avoid protein build up between the posterior portion of the lens and the capsule. The posterior surface may also be configured as a plane or meniscus.
Another IOL lens configuration made in accordance with this invention is shown in Fig. 4 and includes a central optic 50, a haptic 52 which fully encircles the optic and one or more struts 54 which attach the haptic to optic 50. Optic 50 is further provided with identical correction ring zones to the IOL shown in Fig. 1.
Referring now to Figs. 5 and 6, an IOL is shown wherein the diameter of central distance correction zone 12 is increased to approximately 2.1mm. This enables the maximum amount of distance corrected light to enter the eye under extreme bright light conditions and preserves the best available distance vision under the circumstances. It can be appreciated that the diameter of the distance correction zone 12 still has a diameter less than the average pupil diameter (3.0mm) under moderate light conditions and provides true bifocal action.
From an examination of the IOL's shown in Figs. 1 and 2 and Figs. 5 and 6, it can be seen that in each, a greater percentage of lens area is devoted to distance vision than near vision. This is especially important in low light (dark) conditions, where it is desired to maximize the light gathering distance correction surface area. A plot is shown in Fig. 7, of the percentage of area available for distance and near correction under various pupil diameters for a lens incorporating the invention, e.g., such as the lens of Figs. 5 and 6. Under most conditions, except for moderate light conditions which are optimal for reading (pupil diameters 3.0-4.0mm), more than 50% of the IOL's light gathering surface area exposed by the pupil is devoted to distance correction. This assures maximum user safety while providing good light gathering capabilities for reading.
It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. For instance, while the IOL of this invention has been shown implanted in the posterior lens capsule, it may also be implanted in the anterior chamber or in the iris plane. Furthermore, while only two haptics are shown, more may be employed (e.g. three or four) or the lens can be made as a disk which is either flexible, rigid or a combination thereof. In such latter construct, the diameter of the disk is adjusted to mate with the internal dimensions of the posterior lens capsule.
While the ring correction zones have been shown as arranged on the anterior surface of the optic, it is also contemplated that the ring zones may be on the posterior surface of the optic or there could be a combination of rings on the posterior and anterior surfaces which, in combination, provide the desired corrections. If all or some of the ring zones are resident on the posterior surface of the optic, the anterior surface may be concave, plane or convex.
Further, the lens may be constructed of multiple pieces with the haptic constructed from material the same as the optic (e.g., PMMA) or a different material (e.g., polypropylene). These materials are permanently attached to the optic using suitable attachment means.
Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.